Terrestrial stations for spacecraft communications typically include a large aperture antenna for communicating with a spacecraft. Such an antenna typically includes a beam waveguide assembly having a main reflector and a sub-reflector centered on an optical axis of the main reflector, e.g., a Cassegrain antenna. The beam waveguide assembly forms and directs a reciprocal pair of main antenna beams along the optical axis. The main antenna beams typically include a transmit beam for transmitting an uplink signal to and a receive beam for receiving a down-link signal from the spacecraft. To track the spacecraft, the main reflector and the sub-reflector, which are fixed relative to each other and rotate together, along with other optical components of the beam waveguide assembly, are typically driven by motors and servo-mechanisms in at least two rotational directions, e.g., azimuth (AZ) and elevation (EL), so as to align the main beams with the spacecraft. In this manner, the receive and transmit beams are both aligned with the same position of the spacecraft at a given point in time.
A Cassegrain antenna of sufficiently high gain to track a distant spacecraft includes large and correspondingly heavy beam waveguide components, e.g., a main reflector thirty-five meters in diameter, thus necessitating correspondingly bulky and relatively complex motors and servo-mechanisms to rotate such heavy components. Antenna beam tracking accuracy, i.e., alignment accuracy between the main beams and a tracked spacecraft position, is critical when using such a high gain antenna because even a small alignment error, e.g., on the order of millidegrees, results in a significant reduction in peak antenna gain. This criticality is even more pronounced when the antenna is used to track an interplanetary spacecraft because a signal communicated between such a distant spacecraft and the antenna experiences substantial propagational attenuation, i.e., signal attenuation proportional to the square of the distance between the antenna and the spacecraft.
Although the conventional antenna arrangement described above may suffice for communicating with a spacecraft relatively near to the earth, e.g., occupying low, medium and high earth orbits, its use for communicating with a relatively distant, e.g., interplanetary, spacecraft is limited and problematic. Effective communication with the relatively distant spacecraft is complicated in part by a phenomenon referred to as planetary aberration--the phenomenon by which objects in space, as viewed from the earth, are not where they appear to be. Planetary aberration arises as a result of 1) a component of relative motion between the spacecraft and the antenna, specifically, a component of the spacecraft's velocity orthogonal to a line-of-site between the spacecraft and the antenna, and 2) the finite time taken for the uplink and down-link signals to travel between the spacecraft and the antenna due to the finite speed with which the signals propagate through space. The finite time taken for the uplink and down-link signals to travel round-trip between the spacecraft and the antenna is referred to as the round-trip light travel time (RTLT).
The effect of planetary aberration can be appreciated in view of an astronomical coordinate system referred to as the right ascension (RA) and declination (DEC) coordinate system. RA/DEC coordinates define a position on what is referred to as a celestial sphere. The celestial sphere is a two dimensional projection of the sky on a sphere--the celestial sphere--surrounding the earth. Planetary aberration arises because the spacecraft moves in the RA/DEC coordinate system, and thus changes its position over time on the celestial sphere as observed from a point fixed on the earth, i.e., the antenna. The spacecraft changes its RA/DEC position because of its component of orthogonal velocity, without which the spacecraft would tend to maintain a single RA/DEC position and thus move directly toward or away from the antenna.
As will become apparent from the following example, compensating for planetary aberration in the receive and transmit beam tracking of the spacecraft requires an angular separation between the receive and transmit beams. The conventional beam waveguide antenna system disadvantageously includes colinearly aligned receive and transmit beams, i.e., receive and transmit beams aligned in the same direction, and is without a mechanism for imposing such angular separation between the receive and transmit beams, i.e., for splitting the receive and transmit beams apart to compensate for planetary aberration.
The following example serves to illustrate the detrimental effect planetary aberration has on communication between the spacecraft and the colinearly aligned receive and transmit beams of the conventional antenna. Assume a spacecraft initially transmits a down-link signal from a past or previous spacecraft position, and in the finite time taken for the down-link signal to travel to the antenna, i.e., half a RTLT, the spacecraft moves to a present spacecraft position at a present time. Assume at the present time the receive beam of the antenna, along with the optical axis and transmit beam, is aligned with the past spacecraft position to receive the down-link signal arriving therefrom, and, contemporaneous with the arrival of the down-link signal, an uplink signal is transmitted from the antenna via the transmit beam. Assume also in the finite time taken for the uplink signal to arrive at the past spacecraft position, i.e., half a RTLT, the spacecraft moves from the second spacecraft position to a future spacecraft position, i.e., in one RTLT, the spacecraft moves from the past spacecraft position, through the present spacecraft position, and on to the future spacecraft position.
For a relatively near spacecraft, one RTLT is relatively short, e.g., fractions of a second, and the displacement of the spacecraft in RA/DEC coordinates between the past and future positions is negligible with respect to the beam coverage of the receive and transmit beams. Consequently, effective communication can occur even though the uplink signal is transmitted toward the past spacecraft position, and not along a direction intersecting the future spacecraft position, because both spacecraft positions are covered by the transmit beam.
On the other hand, for a relatively distant spacecraft, the one RTLT is relatively large, e.g., 160 minutes for a spacecraft near the planet Saturn, thus leading to an appreciable spacecraft displacement between the past and future spacecraft positions. In this case, the transmit beam coverage does not necessarily encompass the more widely separated positions, a situation worsened by the requirement for a highly directive, i.e., high gain, antenna beam. Without some form of correction or compensation to account for the separation of positions due to planetary aberration, signal loss can be significant, e.g., up to 25 dB. This is due to the colinear alignment of the receive and transmit beams of the antenna with past, present or future positions of the spacecraft. Consequently, ineffective communication results since the uplink signal is transmitted toward the incorrect spacecraft position (e.g., the past position), as a result of this colinear alignment of the receive and transmit beams of the antenna.
For the relatively distant spacecraft, effective communication thus requires simultaneous alignment of the down-link and uplink signals with the respective past and future positions of the spacecraft at the present time, i.e., simultaneous alignment of the receive and transmit beams with respective spaced-apart spacecraft positions coinciding with times half a RTLT previous to and half a RTLT after the present time. Conventionally, achievement of such spaced alignment disadvantageously requires two antennas--one antenna providing receive beam tracking of the past position, and the other antenna providing transmit beam tracking of the future position--because of the colinear receive and transmit beam arrangement of the conventional antenna.
Accordingly, there is a need for a high-gain beam waveguide antenna having a beam steering capability independent of and in addition to the conventional rotational mechanisms used for antenna beam steering.
There is also a need for a high-gain beam waveguide antenna having receive and transmit main beams independently steerable with respect to each other and the optical axis of the antenna.
There is a further need in a beam waveguide antenna system to control the receive and transmit beam tracking of a spacecraft moving along a space trajectory to compensate for appreciable planetary aberration.
There is an even further need for using a single antenna system forming receive and transmit beams to beam-track a spacecraft moving along a spacecraft trajectory to compensate for planetary aberration.
There is also a need to reduce the effects of propagational attenuation of a signal transmitted between a spacecraft and an antenna system.